What are the key differences in physical examination between cellulitis and necrotizing fasciitis?

Comparing Cellulitis and Necrotizing Fasciitis in Physical Examination

The key distinguishing feature between cellulitis and necrotizing fasciitis on physical examination is severe pain disproportionate to clinical findings in necrotizing fasciitis, along with a hard, wooden feel of subcutaneous tissue extending beyond visible skin involvement. 1

Key Distinguishing Features

Cellulitis

• Localized erythema, warmth, and swelling that typically responds to antibiotic therapy 2
• Pain proportionate to visible inflammation 1
• No extension of edema or tenderness beyond the visible erythema 1
• Absence of crepitus or skin necrosis in early stages 2
• Soft tissue consistency remains normal 1

Necrotizing Fasciitis

• Severe pain disproportionate to clinical findings (cardinal early sign) 1
• Hard, wooden feel of subcutaneous tissue extending beyond the area of apparent skin involvement 2
• Edema or tenderness extending beyond cutaneous erythema 1
• Systemic toxicity, often with altered mental status 2
• Failure to respond to initial antibiotic therapy 2
• Bullous lesions, skin necrosis, or ecchymoses (later signs) 2
• Crepitus (indicating gas in tissues) in some cases 1

Evolution and Progression

• Cellulitis typically shows gradual improvement with appropriate antibiotic therapy 2
• Necrotizing fasciitis shows rapid progression despite antibiotics, with worsening systemic toxicity 2
• Many cases of necrotizing fasciitis begin as cellulitis but fail to respond to treatment 2

Diagnostic Challenges

• Early necrotizing fasciitis may be difficult to distinguish from cellulitis due to overlapping superficial features 2
• The most important diagnostic element is clinical judgment 2
• "Pain out of proportion" is a critical early warning sign that should prompt consideration of necrotizing fasciitis 3
• Dramatically elevated CRP levels (5-fold higher than in cellulitis) may help differentiate necrotizing fasciitis 3

Definitive Diagnostic Features

• In necrotizing fasciitis, surgical exploration reveals:
  • Swollen, dull gray fascia with stringy areas of necrosis 2
  • Thin, brownish exudate (not frank pus) 2
  • Extensive undermining of surrounding tissues 1
  • Tissue planes that can be easily dissected with a gloved finger or blunt instrument 2

Adjunctive Diagnostic Tools

• Ultrasound findings in necrotizing fasciitis include:

  • Irregularity of the fascia 4
  • Abnormal fluid collections along the fascial plane 4
  • Diffuse thickening of the fascia compared to the normal limb 4

• CT scan in necrotizing fasciitis shows:

  • Fascial thickening and edema 1
  • Sometimes gas along fascial planes 5
  • Lack of fascial enhancement on contrast CT 5

Clinical Pitfalls to Avoid

• Waiting for classic skin changes (bullae, necrosis) before considering necrotizing fasciitis diagnosis, as these are late findings 1
• Relying solely on imaging to rule out necrotizing fasciitis when clinical suspicion is high 1
• Failing to monitor the progression of cellulitis closely, as subacute forms of necrotizing fasciitis may develop during treatment 6
• Delaying surgical consultation when necrotizing fasciitis is suspected 1

Practical Approach to Differentiation

1. Assess pain intensity relative to visible inflammation 1
2. Palpate beyond the erythema margin for wooden hardness or edema 2
3. Mark the borders of erythema to monitor progression 6
4. Check for systemic toxicity signs disproportionate to visible infection 2
5. Consider surgical exploration for definitive diagnosis when suspicion is high 2
6. In doubtful cases, perform a small exploratory incision in the area of maximum suspicion 2